Almost all dual suspension bicycles these days are non-URTs. The following comments will apply to these designs, as well as the mono-pivot-equivalent i-Drive, made by the GT bicycle company.
The most important thing to look at in assessing any non-URT frame's potential performance is the path that the rear wheel axle travels relative to the main triangle, as the suspension compresses (the main triangle is defined by the seat, handlebars, and bottom bracket). The mechanism that produces the path is not important beyond helping one determine what the path is. In particular, mechanisms that produce similar paths will perform similarly (“suspension rate” aside).
At any moment in time, the rear axle path tangent will primarily determine suspension performance. In particular, within any small segment of any non-URT suspension's travel, that suspension will behave like some mono-pivot with main pivot that gives the same path tangent.
Suspension rate (spring stiffening) is also significant to suspension performance and is determined for the most part by the paths that the shock mounts travel relative to the main triangle. Shorter travel suspensions tend to have higher rates that increase more drastically as the suspensions move through travel (in part due to the fact that many use air shocks these days). However, most frames mate well with their stock shocks, and all common suspension types can achieve the really useful rates (linear or rising). So rate is only a real issue for those wishing to swap different shocks in and out of a given frame.
All this is not to say that all non-URTs will behave the same. Different geometries will certainly have different characteristics. But this depends almost entirely on the specified component paths. It does not depend on the suspension type.
For example, 4-bar “A” may have a rear axle path curvature substantially different from 4-bar “B”, yet A's path may be very close to mono-pivot “C” (circular, with a particular radius and center). Under pedaling and shock absorption, A and C will perform similarly to each other, but differently from B (suspension rate aside).
Almost all non-URTs on the market today have circular rear axle paths out to two or three decimal places, in inches. As a result, the radius and center of curvature primarily determine suspension performance in most non-URTs.
The majority of rear axle paths are of similar radius to conventional mono-pivots. The 4-bar paths plotted in “Typical Horst Link Designs”, which encompass most of the major chain stay pivot design configurations, are all very circular and of conventional radius.
The Giant NRS, The Rocky Mountain ETS-X70, and Cannondale Scalpel have tight radii of curvature, centered inside the rear wheel radius. The Giant NRS and The Rocky Mountain ETS-X70 achieve this through their link configurations. The Scalpel achieves this through the localized bending of its chainstays, about half way between the BB and rear axle. The Scalpel mimics a design we proposed some time ago, called the split-pivot mono. “Soft-tail” designs also have tight radii of curvature, but we consider the length of travel too short for this consideration to be of significance in these designs.
4-bar designs with closely spaced pivots near the frame center can achieve significantly variable path curvature. At the moment, the “The Virtual Pivot Point (VPP)” concept, conceived by Outland and soon to be re-introduced by Santa Cruz and Intense, is unique among viable concepts in its capability to produce significantly variable curvature. However, current examples do not take any real advantage of the possibilities.
Closely spaced 4-bar pivots can also achieve wider curvature then is possible in a mono-pivot. The positive travel sections of the current “The Virtual Pivot Point (VPP)” bikes contain such a curvature and the Schwinn Rocket 88 also claims such.
However, closely spaced pivot locations near the highly stressed bottom bracket area may come with a larger tradeoff between weight and durability, as the links and pivots in this area must be more heavily built.
One unique frame design, which we have yet to evaluate, is the Maverick. At the moment, it is very expensive and still hard to find. We hope to include it in these pages soon.